It's very simple, really. Back in the early 70s, many Presbyterians noticed their denomination starting to lose focus and ignore doctrine for the sake of politics. So, they left - and got back to the roots of Reformed theology. When the solid core left, the remnant became a shambles.
The same thing happened to the Lutherans. In the mid-70's the big church was the LCMS and it split when the liberals in that body walked away with about a third of the membership and merged with several smaller Lutheran bodies to form the ELCA. The really tragic thing is that at the time the bulk of them were still pretty solid, but there were some real lefties in the mix who took the church in a scary direction. At the same time, the remnant in the LCMS (which was a very solid body doctrinally) got a bit reactionary and, I believe have OVER-reacted to the point of being too insular and losing site of the Great Commission. In total it's been a tragedy for Lutheranism in North America as a whole.